An experimental scramjet-powered, ultrahigh speed strike vehicle is emerging as the Pentagonâ€™s main choice for a new long-range, rapid attack weapon, a senior Pentagon official says.

Alan R. Shaffer, principal deputy assistant defense secretary for research and engineering, told a defense industry conference that prototypes and recent tests proved concepts for hypersonic arms, and several systems are part of a high-priority effort by Pentagon weapons developers, despite the era of sharply-diminished defense spending.

...the senior weapons research official said the Pentagonâ€™s most promising hypersonic vehicle is the X-51, a cruise missile-sized weapon powered by an advanced engine called a scramjet. The X-51, developed by Boeing, flies at up to 3,882 mph, or Mach 5.1, and is launched from under the wing of a B-52 bomber.